Moved the add_timer, fire_timer_once, and cancel_timer functions to timer.hpp and timer.cc.
Renamed timer_provider_interactor_t to timer_provider_callback_t.
Moved timer_token_t definition to timer.cc.
Made writeback_t::on_timer not have that silly "self" variable.
Added comment for get_num_db_threads function (that was once present in db_thread_info.cc).